A. V. Kustov

Traveling convection vortices as seen by the SuperDARN HF radars

Two impulsive traveling convection vortex (TCV) events observed simultaneously by ground based magnetometers and the SuperDARN HF radars in the prenoon sector were studied. In both cases, disturbances traveled westward at speeds of 4–6 km/s. Convection patterns derived from magnetometer measurements and radar observations were overall in reasonable agreement; observed differences at some points might be caused by both the nonuniform ionospheric conductivity distribution and difference in the integration time of the radar and magnetometer data. For one event, the convection patterns obtained from magnetometer data and SuperDARN radar measurements were relatively simple; they can be interpreted as a result of the westward motion of a convection vortex system associated with a pair of field-aligned currents separated in azimuthal direction. This TCV event was associated with relatively low Pc5 pulsation activity, contrary to the second TCV event that was accompannied by a train of Pc5 magnetic pulsations of large amplitude. Convection patterns for the second event were complicated. A simple scenario for the interpretation of the generation of TCVs and Pc5 pulsations is suggested. A sudden impulse in the solar wind dynamic pressure produces disturbances on several boundaries of magnetospheric plasma: on the magnetopause, the LLBL inner edge, and the plasma sheet inner edge. These boundaries are elastic so that surface waves can propagate along them. The high-latitude wave is responsible mainly for TCVs, whereas the low-latitude waves may be responsible for excitation of Pc5 field line resonance pulsations. The scenario explains important features of both TCV events and Pc5 pulsations: both phenomena appear simultaneously and show westward (eastward) propagation, but the TCVs are observed at latitudes close to the LLBL inner edge, whereas the Pc5 pulsations occur at lower latitudes, close to the inner boundary of the plasma sheet.

Aspect angle dependence of the radar aurora Doppler velocity

The aspect angle dependence of the Doppler velocity of radar auroral echoes was studied by means of a comparison between the signals received simultaneously on four 50-MHz CW links. In most of the events, the observed Doppler velocity decreased with increasing aspect angle but the rate of decrease was lower than that predicted by linear theory. The velocity decrease was about a factor of 0.4 between geometrical aspect angles of 1.5° and 4.0° and 0.6 to 0.7 between 3.2° and 5°. A model for the Doppler velocity behavior of the radar auroral signals is proposed, combining both propagation and scattering effects. Allowance is made for “ray spreading” arising from processes like refraction and diffraction during propagation. In addition, height integration over the scattering volume is taken into account.

Field‐aligned currents in the polar cap at small IMF Bz and By inferred from SuperDARN radar observations

Routine SuperDARN observations of the ionospheric plasma convection and field-aligned currents (FACs) in the high-latitude ionosphere are used to study current systems established at small interplanetary magnetic field (IMF) B z and B y , By statistical averaging of available data sets we show that under this IMF condition the ionospheric convection pattern consists of two (evening and morning) convection cells that are similar in shape. The flow intensity inside the central polar cap is noticeably depressed so that plasma entering the polar cap flows around its border, predominantly along the lines of equal magnetic latitude, so that the convection cells are of a crescent-like shape. This global pattern of plasma flow is associated with the effect of the region 0 field-aligned currents coexisting with the region 1 and region 2 field-aligned currents. SuperDARN observations of FACs for individual events support this conclusion. FACs were derived by analyzing the vorticity of the SuperDARN convection maps. We show that region 0 currents for small IMF B z and B y can exist in time sectors way off the magnetic noon. Thus radar observations support earlier findings from satellite magnetometer measurements of the region 0 current system at high latitudes during both the prenoon and afternoon at small IMF intensities. Because the region 0 FACs occur during small IMF intensities, it is suggested that quasi-viscous processes play a role in their generation.

Ionospheric convection and equivalent ionospheric currents in the dayside high‐latitude winter ionosphere

Ionospheric convection inferred from Super Dual Auroral Radar Network (SuperDARN) HF radar measurements is compared with an equivalent ionospheric convection derived from ground magnetometer data in the dayside winter high-latitude ionosphere. Although there was general agreement between observed convection patterns produced by radars and magnetometers, there were significant differences in details. The orientation of equivalent convection vectors inferred from magnetic data was often opposite to the convection vectors determined by the SuperDARN radars in the poleward part of the convection vortex structure, though the agreement was reasonable in its equatorward part. The magnitudes of convection vectors determined from radar data and those inferred from magnetometer data were often different. The observed differences are attributed to strong horizontal inhomogeneity in the ionospheric conductivity distribution for winter conditions. It is possible that magnetic disturbances in the dark high-latitude ionosphere are strongly affected by field-aligned currents at the terminator that separates regions of the sunlit highly conducting ionosphere and dark poorly conducting ionosphere.

Relationship of the SAPPHIRE‐North merged velocity and the plasma convection velocity derived from simultaneous SuperDARN radar measurements

Data taken during joint observations using the 50-MHz Saskatchewan Auroral Polarimetric Phased Ionospheric Radar Experiment (SAPPHIRE)-North radar and the HF Super Dual Auroral Radar Network (SuperDARN) radars are combined to investigate the relationship between the merged velocity derived from SAPPHIRE measurements and the plasma convection velocity inferred from SuperDARN measurements. Only events with broad and smooth type 2-like SAPPHIRE echoes were considered. In magnitude, SAPPHIRE merged velocities were found to be about 3 times less than SuperDARN convection velocities. In azimuth, the merged and convection velocities differed by less than 30° for most of the events. Thus the SAPPHIRE radar system can be used for rough estimations of the plasma convection direction but not the magnitude of the convection.

The SAPPHIRE‐North radar experiment: Observations of discrete and diffuse echoes

Since February 1995, regular observations have been carried out by the University of Saskatchewan radar group using a new 50-MHz Doppler radar system called SAPPHIRE-North. The SAPPHIRE-North radar monitors the ionospheric processes in the transitional region between the auroral oval and the polar cap over Cambridge Bay, Northwest Territories. In this paper, results of the first 2 months of observations are presented. Echoes were registered at about 10% of observational time. Diurnal echo occurrence exhibited two distinct maxima, one close to noon and another prior to midnight. The noon echoes were usually of a diffuse type, while midnight echoes were of a discrete type. Midnight discrete echoes were found to be associated with an arrival of poleward expanding substorm bulges into the SAPPHIRE-North field of view. Noon diffuse echoes were most likely due to electric field enhancements in the vicinity of the cusp/cleft region. Salient spectral features of both the diffuse and discrete echoes are presented and compared with auroral zone results. The role of ionospheric refraction for observations of these high-latitude echoes is discussed.

Spectral width of type 2 coherent echoes at large magnetic aspect angles

The spectral characteristics of smoothly varying type 2 signals were studied using two 50-MHz CW radio links. The echoes observed came from two closely located scattering volumes where the geometrical aspect angles of observations were about 10° for both links but the azimuthal angles differed by 88°. The events reported here were selected when the spectra on both links were wide, with a comparable spectral width, but the mean Doppler shift was almost zero on one link and very high, sometimes in excess of the ion-acoustic velocity, on the other link. Highly shifted Doppler spectra were observed to be more skewed than the spectra with low shifts. The skewness was negative for negative Doppler shifts and positive for positive Doppler shifts in accordance with previous radar observations at large magnetic aspect angles. The variation of spectral width with the radial velocity or with the magnetic disturbance level exhibited two regions of linear dependence, with a larger slope at small values and a much smaller slope at higher ones. These different slopes likely reflect the change in the rate of the irregularity amplitude increase with the electric field.

Super Dual Auroral Radar Network observations of near‐noon plasma convection at small interplanetary magnetic field Bz and By

Routine Super Dual Auroral Radar Network ionospheric plasma convection maps are used to study the structure of the near-noon convection established at small interplanetary magnetic field (IMF) Bz and By. During such intervals, the influence upon convection of the quasi-viscous processes at the magnetopause is expected to be at least as important as the effect of magnetic field line reconnection processes. It was found that, despite the small IMF Bz and By, there was a significant plasma flow across the noon meridian, so that the convection throat with the antisunward plasma flow was shifted toward earlier magnetic local times (to ∼1100 magnetic local time). It was also found that the size and shape of the morning convection cell was similar to the size and shape of the evening cell. It is argued that the shift of the convection throat toward morning hours is most likely caused by the effects of the ionospheric polar cap plasma disconnection from corotation with the ionospheric plasma on closed magnetic field lines. Other possible causes for the observed asymmetry are discussed.

On the spectral asymmetry of type II coherent echoes at large magnetic aspect angles

The asymmetry (skewness) of broad type II Doppler spectra collected by the pre Saskatchewan Auroral Polarimetric Phased Array Ionospheric Radar Experiment (SAPPHIRE) coherent radar system during westward electrojet conditions was studied. The system included two 50-MHz CW links which monitored spatially close regions of the ionosphere with about the same aspect angles of ∼10° but with an azimuth difference of the bisectors of about 90°. Periods of scatter were selected when both radar links recorded echoes with a wide, type II spectrum. Typically, the spectra were asymmetrical on both links and had a magnitude of skewness in between 0.3 and 0.5. For radar links with a positive (negative) mean Doppler shift, the skewness was observed to be positive (negative) contrary to previous reported auroral zone experiments (positive skewness means a longer spectral tail toward higher velocities). The spectra were more skewed for observations along the electrojet, for higher echo power and also for narrower spectral width. The characteristic features of the spectral asymmetry are attributed to the plasma turbulent processes at large off-orthogonal angles.

Electric field and electron density thresholds for coherent auroral echo onset

Simultaneous observations of the Polar Geophysical Institute 83-MHz auroral radar and the European Incoherent Scatter facility were used to study the coherent echo threshold phenomenon. It is shown that, for electric fields not far from the Farley-Buneman instability threshold of ∼20 mV/m, the onset of echo occurrence was due to an electron density increase above some threshold value. This value was estimated to be ∼2.5 × 1011 m−3. For smaller electric fields of 5-10 mV/m, echoes were not observed even for electron densities double the above threshold value. The importance of both factors, electric field and electron density, and some other effects are discussed in light of recent studies of coherent echo intensities and their dependence on plasma conditions at electrojet altitudes.